AlGaN/GaN type high electron mobility transistors (HEMTs) that achieve high power operation, high-speed switching, and high-temperature operation have been drawing attention as field-effect transistors that may be used for a portable base station and satellite communication, etc.
Many conventional GaN type HEMTs are normally-on types. In the normally-on type GaN type HEMTs, current continues to flow when a failure occurs. Thus, in order to ensure safety at a failure, a normally-off GaN type HEMT is desirable. To this end, a GaN HEMT with a recess gate structure (for example, refer to FIG. 1) has been proposed to obtain a positive threshold voltage. In a GaN type HEMT illustrated in FIG. 1, a protective layer 104 including n-type GaN is formed over an electron supply layer 103 including n-type AlGaN. An AlN layer 105 with a recess (an opening) is formed over the protective layer 104. A SiN film 106 is formed over the AlN layer 105. A gate electrode 111g is formed over the recess and a source electrode 111s and a drain electrode 111d are formed over the protective layer 104 so that the gate electrode 111g is located between the source electrode 111s and drain electrode 111d. In this technique, intentionally, no insulator layer is sandwiched between the protective layer 104 and the gate electrode 111g in order to prevent a threshold voltage becoming high.
Although this structure allows a normally-off operation, it is difficult to suppress a phenomenon called a current collapse caused by concentration of an electric field.